1. Field of the Invention
The invention relates to a secondary air supply system for an internal combustion engine.
2. Description of Related Art
There is known an exhaust gas control system in which an exhaust gas purification catalyst is arranged in an engine exhaust passage of an internal combustion engine and then components of carbon monoxide (hereinafter, referred to as “CO”), hydrocarbons (hereinafter, referred to as “HC”) or nitrogen oxides (hereinafter, referred to as “NOX) in exhaust gas are purified.
There is known a secondary air supply system in which secondary air is supplied into an exhaust pipe to increase an oxygen concentration by feeding air under pressure from an air pump to a secondary air supply passage connected to an engine exhaust passage at a portion upstream of such an exhaust gas purification catalyst and, by so doing, HC and CO in exhaust gas are oxidized, and accordingly purification of exhaust gas is facilitated. In a related art, there is suggested a secondary air supply system that is able to estimate the flow rate of secondary air that is supplied to an engine exhaust passage.
Japanese Patent Application Publication No. 2005-163709 (JP 2005-163709 A) describes a secondary air supply system that includes a control valve and a pressure sensor. The control valve is able to open and close a secondary air supply passage that is provided downstream of a pump that supplies air. The pressure sensor is arranged between the pump and the control valve. JP 2005-163709 A describes that a pressure in the secondary air supply passage is measured at the time when the control valve is open and at the time when the control valve is closed and then the flow rate of air that flows through the secondary air supply passage at the time when the control valve is open is calculated using these measured pressures.
As for a driving control device for an actuator that drives a predetermined driving target device, Japanese Patent Application Publication No. 2010-209785 (JP 2010-209785 A) describes a driving control device for an actuator. The driving control device includes: a reference voltage output unit that outputs a reference voltage; a supply voltage output unit that amplifies a voltage or a current and that outputs a supply voltage on the basis of the reference voltage; a sensor that is supplied with the supply voltage and that outputs a detected signal based on an operation amount of an operating target; and control means for calculating a correction signal by multiplying the detected signal by a value obtained by dividing the reference voltage by the supply voltage and then executing drive control over the actuator on the basis of the correction signal. JP 2010-209785 A describes that the driving control device is able to execute drive control over the actuator without receiving the influence of fluctuations in the supply voltage even when the supply voltage to the sensor fluctuates due to a high temperature.
Japanese Patent Application Publication No. 2008-215129 (JP 2008-215129 A) describes a control system for an internal combustion engine. The control system includes: an air flow meter that receives a power supply voltage from a direct-current power supply; a voltage sensor that detects the power supply voltage; and a control unit that controls a parameter that causes a change in intake air flow rate, and that controls a fuel injection amount that is supplied to an engine on the basis of a signal that indicates the intake air flow rate detected by the air flow meter. The control unit controls a throttle opening degree so as to limit the intake air flow rate in response to a decrease in the power supply voltage when the power supply voltage detected by the voltage sensor becomes lower than a first voltage. JP 2008-215129 A describes that the control system for an internal combustion engine is able to maintain engine operation close to a normal state by suppressing an error of the air flow meter even when the power supply voltage decreases.
When a secondary air supply system for an internal combustion engine is used for an extended period of time, fine particles contained in intake gas taken in from an intake system of the internal combustion engine and fine particles produced from lubricated portions and driving portions of an air pump adhere to an inner wall, or the like, of a secondary air supply passage, and accumulate as a deposit. Fine particles in exhaust gas also adhere to the inner wall, or the like, of the secondary air supply passage, and accumulate as a deposit. In this case, the pressure in the secondary air supply passage increases because the pipe resistance of the secondary air supply passage increases. When a secondary air flow rate is estimated from the pressure in the secondary air supply passage, it may be erroneously determined that the secondary air flow rate has increased due to accumulation of a deposit although the secondary air flow rate has actually decreased.
Secondary air is supplied to an exhaust system of the internal combustion engine via the secondary air supply passage by actuating an air pump. When the air pump is used for an extended period of time, a pump discharge capacity gradually decreases with aging degradation. Therefore, when the secondary air supply system is continuously used, the flow rate of air that can be supplied to an engine exhaust passage gradually decreases.
The secondary air supply system described in JP 2005-163709 A is able to estimate a secondary air flow rate in consideration of both the influence of a deposit that adheres to the inner wall of a secondary air supply pipe and a decrease in the discharge pressure of the air pump due to aging degradation.
In this secondary air supply system, the pressure in the secondary air supply passage during valve shutoff operation, that is, when the control valve arranged downstream of the air pump is closed, is detected in order to estimate the secondary air flow rate. At the time of a start of valve shutoff operation, that is, at the time when the control valve is closed, the pressure in the secondary air supply passage increases, and then becomes substantially a constant value after a lapse of a predetermined period of time.
There is a case that the secondary air supply system desirably early opens the control valve to supply secondary air to the engine exhaust passage. In this case, before the pressure in the secondary air supply passage becomes substantially constant, the control valve is opened, and secondary air is supplied to the engine exhaust passage. For example, at the time of a cold start of the internal combustion engine, it is desirable to start supplying secondary air as early as possible in order to early increase the temperature of the exhaust gas purification catalyst. It is desirable to open the control valve at the time of a transitional state before a steady state, in which the pressure in the secondary air supply passage is substantially constant, after the air pump is started. However, because a pressure is detected in the transitional state during valve shutoff operation in which the control valve is closed, when the secondary air flow rate is estimated using the detected pressure, the estimated secondary air flow rate may have an error.
When the internal combustion engine is arranged in a vehicle, or the like, an electrical load device that is an electrical load is connected to a battery. The air pump that supplies air in the secondary air supply system is the electrical load device, and is actuated by electric power that is supplied from the battery. Another electrical load device other than the air pump may be connected to the battery. When the amount of electric power that is consumed by the other electrical load device is large; the output voltage of the battery may decrease as the other electrical load device is operated. When the secondary air flow rate is detected in the case where the output voltage of the battery has decreased and accordingly the driving voltage for the air pump has decreased, the detected secondary air flow rate may have an error.
In the secondary air supply system described in JP 2005-163709 A, a transitional state at the time when such valve shutoff operation is performed, an operation state of another electrical load device, or the like, is not taken into consideration, so there is room for improvement.